Thermoplastic resins have excellent processability and a high degree of freedom in shape, and therefore are widely used in automobiles, electrical devices, medical and biological devices, and other general industrial applications, and are commonly used to such an extent that there is no field in which thermoplastic resins are not used and have become familiar materials. At the beginning, thermoplastic resins are used as substitutes for natural materials such as wood materials and paper, however, today, a number of specialized products which can be produced only with a plastic material have been developed. Due to this, if an optimal material or an optimal processing method can be provided for the purpose of design development, there is a possibility to produce unprecedented novel products.
In addition, due to the recent trend toward limits on carbon dioxide emissions and reduction in costs, the functions of thermoplastic resins are enhanced, and also thermoplastic resins are gradually substituted for metals. However, thermoplastic resins generally have many inferior properties to metals, for example, thermoplastic resins have a low heat-resistant temperature, a low mechanical strength, and a high thermal expansion, are easily deformed or decomposed, easily dissolve in an organic solvent, and are easily swollen by water, etc., and therefore, it is impossible to completely substitute thermoplastic resins for metals.
In particular, since the structures of recent products become complicated, designing is made by utilizing the respective advantages of thermoplastic resins and metals, and therefore, a secondary processing technique therefor becomes important. Among these, studies of methods using a laser have been increasing recently.
In PTL 1, it is described that by irradiating an acrylic resin and tin with an irregular surface which has been roughened with sandpaper with a laser in a state where the both members are in close contact with each other, the acrylic resin gets into the surface irregularities to form strong bonding.
In PTL 2, it is shown that by irradiating a molded body composed of a thermoplastic resin and a metal with a laser from the metal side in a state where both members are overlapped with each other, the molded body can be strongly bonded to the metal even if the laser light does not transmit through the molded body. It is also described that a surface treatment of the surface of the metal on the bonding surface side is effective in the improvement of the bonding strength.
In NPL 1, it is shown that by irradiating a plastic and a metal with a high-power laser in a state where both members are overlapped with each other, minute air bubbles are generated in the vicinity of the interface of the plastic, and by the effect of the pressure at the time of generating the air bubbles, the plastic and the metal can be strongly bonded to each other.